A solid oxide fuel cell (SOFC) stack assembly is the power-producing component in an SOFC electric power plant such as an auxiliary power unit (APU) for a vehicle, a stationary power generating unit (SPU), a combined heat and power unit (CHP), or other such system. In a practical and manufacturable SOFC power system, the stack assembly typically is manufactured as a stand-alone component mounted into the power system for ease of assembly, service, and replacement. The power system provides fuel gas to the anode side of the stack, and provides air as an oxidant and coolant for excess heat removal to the cathode side of the stack. Partially depleted fuel gas is removed from the stack for use elsewhere in the power system. The SOFC stack must be maintained at an operating temperature between 650° C. and 1000° C., and preferably between 750° C. and 800° C.
The fuel gas and cathode air typically are fed to and removed from the stacked individual fuel cells by integral gas distribution channels within the stack known in the art as “chimneys”. The chimneys are carefully designed to distribute the gases evenly to the anode and cathode gas cavities of each fuel cell unit in the stack. The gases entering and exiting the stack must also be routed in such as way that they are properly distributed to the chimneys to assure even flow distribution across the surfaces of each cell within the anode and cathode gas cavities.
A stack must be easily and reliably mounted to, and removable from, a system manifold with a good seal assuring minimal leakage of air and/or fuel gas. In addition, for proper sealing of the multiple layers in a stack, a compressive load must be maintained within the stack at all times.
In the prior art, these functions have been achieved by a specific arrangement wherein the stack is mounted to a base plate which in turn is mounted onto a system manifold. The base plate has openings in it that align with the chimneys as well as with openings in the system manifold. The distribution of gases to the chimneys is determined by the configuration and design of the system manifold. See, for example, U.S. Pat. No. 6,967,064 B2 and US Patent Application Publication No. US 2003/0235751 A1. The stack is sealed to the base plate by a high-temperature adhesive seal, and the base plate is sealed to the system manifold by a compressive high-temperature gasket.
In this prior art arrangement, the compressive loading mechanism must provide load not only for integrity of the stack layers but also through the stack to maintain a much higher compressive sealing load on the base plate gasket. There are multiple drawbacks to this design.
First, the base plate, fabricated from ferritic stainless steel to match the coefficient of thermal expansion (CTE) of the fuel cell stack components, must be very thick and massive to attempt to maintain a uniform compressive load on the gasket.
Second, the system manifold must have sufficient structural rigidity to attempt to maintain a uniform compressive load on the gasket against the base plate, requiring expensive alloys for providing such rigidity at the high SOFC operating temperatures.
Third, the stack compressive loading mechanism must provide more load than is required for stack seal integrity in order to provide sufficient load for the gasket, and is therefore heavier duty and dimensionally larger than would otherwise be necessary.
When a prior art SOFC power system is constructed to account for all these considerations, distribution of gases to the chimneys may still be suboptimal due to the system manifold requirements; further, leaking of the compressive gasket may still occur with unacceptable frequency, allowing combustible gas to mix with hot air, resulting in localized overheating or system failure.
Another requirement for the SOFC stack is that the fuel gas and cathode air be provided to the stack at similar temperatures (within approximately 25° C.). Depending on the power system configuration and operating condition or mode, the anode gas can vary over a wide range and not match the controlled temperature of the cathode air. As a result, a heat exchanger function has been added to the power system to equalize the anode and cathode gas stream temperatures (reference U.S. Pat. No. 6,967,064 B2). The problem with the prior concepts is that they were discrete devices which added to the packaging complexity and cost of the system.
What is needed in the art is a design and assembly arrangement for an SOFC stack and manifold that prevents leakage between the stack and the manifold and reduces the compressive loading requirement on the stack.
It is a principal object of the present invention to prevent leakage of fuel gas and/or cathode air from between an SOFC stack and a system manifold.
It is a further object of the invention to reduce the weight, size, cost, and complexity of an SOFC power unit.
It is a still further object of the invention to improve the durability and reliability of an SOFC power unit.